An organic electroluminescence device (hereinafter the term “electroluminescence” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device in the form of a stacked type device, studies on organic EL devices wherein organic materials are used as the constituting materials has actively been conducted.
The organic EL device reported by Tang et al. has a multilayer structure in which tris(8-hydroxyquinolinol) aluminum is used as an emitting layer and a triphenyldiamine derivative is used as a hole-transporting layer. The advantages of the multilayer structure include increased injection efficiency of holes to the emitting layer, increased generation efficiency of excitons generated by recombination by blocking electrons injected from the cathode, confinement of the generated excitons in the emitting layer, and so on.
As the multilayer structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such multilayer structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
Conventionally, aromatic diamine derivatives or aromatic condensed ring diamine derivatives have been known as hole-transporting materials used in the organic EL device.
However, in order to attain sufficient luminance in an organic EL device in which these aromatic diamine derivatives are used as a hole-transporting material, problems such as shortened device life and increased consumption power occur, since an applied voltage is required to be increased.
As the method to solve these problems, a method has been proposed in which an electron-acceptable compound such as Lewis acid is doped to the hole-injecting layer of the organic EL device (Patent Document Nos. 1 to 7, or the like). However, the electron-accepting compound used in Patent Document Nos. 1 to 4 suffers from a problem in which it becomes unstable when handling during the production process of the organic EL device or the device life is shortened due to insufficient stability such as heat resistance at the time of driving of the organic EL device.
In addition, tetrafluorotetracyanoquinodimethane (TCNQF4), which is an electron-accepting compound exemplified in Patent Documents 3 and 5 to 7 or the like, has a small molecular weight and is substituted with fluorine. Therefore, it has a high sublimation property, and may diffuse within an apparatus when fabricating an organic EL device by vacuum vapor deposition, thereby contaminating the apparatus or the device.    Patent Document 1: JP-A-2003-031365    Patent Document 2: JP-A-2001-297883    Patent Document 3: JP-A-2000-196140    Patent Document 4: JP-A-H11-251067    Patent Document 5: JP-A-H4-297076    Patent Document 6: JP-T-2004-514257    Patent Document 7: US2005/0255334A1
The invention has been made based on the above-mentioned problems, and an object thereof is to provide an electron-acceptable material which is suitable as a material constituting an organic EL device.